Q: Translucent gray border around mac screen?

This happens because you have the "Show Desktop" trackpad gesture turned on. The gesture that activated it was to spread your thumb and three fingers. You can undo it by pinching your thumb and three fingers together.

Look under the "More Gestures" tab in the "Trackpad" section of the System Preferences for a video demonstration of how to do it, or scroll down to the "Show Desktop" section of this article: https://support.apple.com/en-us/HT204895

This is usually what the child/pet/alien has done to get the system in that state in the first place.

thanks. you helped me fix it

This helped me too! Thanks. I was a cat on the keyboard victim

Thankyou for this solution. The grey border was driving me mad!

what else can I do in terminal?

thank you so much this worked for me!!

Had the same problem and pvandrunen reply helped a lot! Thanks

ThnXs Buddy.. Good help..

I'm another Baby on the Keyboard Victim

Thanks! this helped fix the issue!

Thank you very much!  This was very helpful and works with ease

my 15 month old did this too! lol!

this did the trick. Thanks

Thank you, it worked for me!!! 

My little brother messed around on my laptop and caused this border to appear.

But its fixed now! Thankyou